This invention relates to fourth generation computed tomography using helical scanning. More specifically, the invention concerns an image reconstruction method for reducing image artifacts that result from acquiring tomographic projection data in a helical scan.
In a fourth generation computed tomography system, an x-ray source is collimated to form a fan beam with a defined fan beam angle. The fan beam is orientated to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane", and to be transmitted through an imaged object to an x-ray detector array orientated within the imaging plane. The x-ray source may be rotated on a gantry within the imaging plane, around the imaged object, so that the fan beam intercepts the imaged object at different angles.
The detector array is comprised of detector elements which each measure the intensity of transmitted radiation along rays projected from the x-ray source to that particular detector element. However, unlike the case of conventional third generation CT scanning where the detector array moves with the source, with a fourth generation system the detector array is fixed and so each detector element receives rays at a variety of angles as the source is rotated around the imaged object.
At each angle of the source, a data is acquired from each exposed detector element. This data, collected at a single source position, will be termed a "source-vertex" projection, and is similar to the projections acquired in third generation CT imaging.
The source is then rotated to a new angle and the process is repeated with the detectors receiving rays at new angles. The data for a given detector over 360.degree. of source rotation will be termed a "detector-vertex" projection to be distinguished from the source-vertex projections described above. The detector-vertex projections are derived from the acquired source-vertex data.
The acquired detector-vertex projection set is typically stored in numerical form for computer processing to "reconstruct" a slice image according to detector-vertex reconstruction algorithms known in the art. The reconstructed slice images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera. The use of detector-vertex projections in the reconstruction process rather than source-vertex projections reduces certain image artifacts associated with varying sensitivities among the detectors.
A typical computed tomographic study entails the imaging of a series of slices of an imaged object with the slices displaced incrementally along a z-axis perpendicular to the x and y axes, so as to provide a third spatial dimension of information. A radiologist may visualize this third dimension by viewing the slice images in order of position along the z-axis, or the numerical data comprising the set of reconstructed slices may be compiled by computer programs to produce shaded, perspective representations of the imaged object in three dimensions.
As the resolving power of computed tomography methods increases, additional slices are required in the z-dimension. The time and expense of a tomographic study increases with the number of slices required. Also, longer scan times increase the discomfort to the patient who must remain nearly motionless to preserve the fidelity of the tomographic reconstructions. Accordingly, there is considerable interest in reducing the time required to obtain a slice series.
The time required to collect the data for a series of slices depends in part on four components: a) the time required to accelerate the gantry to scanning speed, b) the time required to obtain a complete tomographic projection set, c) the time required to decelerate the gantry and d) the time required to reposition the patient in the z-axis for the next slice. Reducing the time required to obtain a full slice series may be accomplished by reducing the time required to complete any of these four steps.
The time required for acceleration and deceleration of the gantry may be avoided in tomographic systems that use slip rings rather than cables to communicate with the gantry. The slip rings permit continuous rotation of the gantry. Hereafter, it will be assumed that the CT systems discussed are equipped with slip rings or the equivalent to permit continuous rotation of over 360.degree..
The time required to acquire the tomographic data set is more difficult to reduce. Present CT scanners require on the order of one to two seconds to acquire the projection set for one slice. This scan time may be reduced by rotating the gantry at a faster speed. A higher gantry speed, in general, will reduce the signal-to-noise ratio of the acquired data by the square root of the factor of rotational rate increase. This may be overcome to some extent in transmission tomography devices by increasing the radiation output of the x-ray tube, but is subject to the power limits of such devices.
A reduction in patient repositioning time may be accomplished by translating the patient in the z-axis synchronously with the rotation of the gantry. The combination of constant patient translation along the z-axis during the rotation of the gantry and acquisition of projection data has been termed "helical scanning" and refers to the apparent path of a point on the gantry with respect to a reference point on the imaged body. As used herein, "helical scanning" shall refer generally to the use of continuous translation of the patient or imaged object during the acquisition of tomographic imaging data, and "constant z-axis scanning" shall refer to the acquisition of the tomographic data set without translation of the patient or imaged object during the acquisition period.
Continuous translation of the imaged object during scanning shortens the total scanning time required for the acquisition of a given number of slices by eliminating the length of time normally required for repositioning the patient between scans. However, helical scanning introduces certain errors with regard to the data in the acquired tomographic projection sets. The mathematics of tomographic reconstruction assumes that the tomographic projection set is acquired along a constant z-axis slice plane. The helical scan path clearly deviates from this condition and this deviation results in image artifacts in the reconstructed slice image if there is any significant change in the object in the z-axis. The severity of the image artifacts depends generally on the "helix offset" in the projection data, measured as the difference between the table locations of the scanned data and the z axis value of the desired slice plane. Errors resulting from helical scanning will be referred to collectively as "skew" errors.
Several methods have been used to reduce skew errors in helical scanning. A first approach disclosed in U.S. Pat. No. 4,789,929 issued Dec. 6, 1988, interpolates between projections of consecutive 360.degree. tomographic projection sets. This approach of interpolating over 720.degree. generally increases partial volume artifacts. Partial volume artifacts are image artifacts arising when certain volume elements of the imaged object contribute to only some of the projections of the projection set. In a second approach, described in co-pending U.S. patent application Ser. No. 07/435,980, filed Nov. 13, 1989 entitled "Extrapolative Reconstruction Method for Helical Scanning", and assigned to the same assignee as the present invention, skew artifacts are reduced by interpolating and extrapolating between two partial projection sets of only 180.degree. of gantry rotation. This application is incorporated by reference.